The present invention relates to a double-sided milling cutting insert for fine machining, which is made by form-pressing and sintering of a powder that constitutes a cutting insert material and comprises two substantially similar main surfaces, which are turned by 90.degree. relative to each other, and four substantially similar side surfaces that extend between these main surfaces.
Such milling cutting inserts are fastened in a way known per se in rotative milling tools. Usually, this fastening is made on insert-carrying cassettes, as for instance described in the Swedish patent application 9300889-4 and in DE-A-4 013 717. In order to achieve the required surface smoothness of the workpiece, it is necessary to locate the cutting insert as exactly as possible in the desired position, particularly in the axial direction. If the axial positioning is unsatisfactory, then so called axial play arises, which causes a deteriorated surface smoothness of the workpiece.
In for instance the motor industry, close pitch milling cutters are used for the milling of engine blocks, cylinder blocks and similar parts. For those applications, very high requirements are set for surface smoothness and R.sub.a -values of maximally 1.5 .mu.m, R.sub.z -values of between 10 and 15 .mu.m and WT-values, of between 5 and 8 .mu.m are often required. It has been possible to attain those tolerances by the construction described in the above mentioned Swedish patent application.
A problem that often arises in fine milling applications is the creation of so called edge chippings, i.e., chips are torn off from the periphery of the produced surface when it is being machined. Of course, edge chippings may also occur around the edges of recesses on the machined surface, such as around the edge of a boring.
Another problem often arises during the milling of thin-walled workpieces. As is known, for the sake of saving material and weight, certain parts of a workpiece may be relatively thin-walled, such as between 3 and 5 mm. These thin portions tend to yield to some extent to the pressure from the milling tool, which in turn results in a certain waviness on the produced surface.
A further problem with several of the fine milling cutting inserts on the market is the fact that they to a high degree require a costly precision grinding in order to attain the highly set tolerance requirements. Of course, this increases the production costs most considerably. As an example of this, one may again mention DE-A-4 013 717, and the known cutting insert reproduced in FIG. 7. Another inconvenience of the inserts disclosed in DE-A-4 013 717 is that they are slightly rhombic. This has the consequence that only two of the four corners on each side may become operatively active. Thus, this disadvantage doubles the already previously considerable production cost per cutting corner.
Thus, a first object of the present invention is to provide a milling cutting insert that reduces the size and the number of edge chippings to a minimum.
A second object of the present invention is to achieve fine and smooth surfaces also on thin-walled workpieces.
Still another object of the present invention is to reduce the axial pressure of the milling tool against the workpiece.
A still further object of the present invention is to minimize the costs for the production of the milling cutting insert.